In a liquid ejection apparatus which ejects liquid through its nozzles by applying pressure on the liquid such as an ink jet printer which ejects ink, air bubbles may enter or be mixed in the liquid channels communicating with the nozzles. The mixed bubbles absorb part of the pressure to be applied on the liquid. This may prevent the liquid from being ejected normally through the nozzles. Therefore, it is necessary to completely discharge the air bubbles mixed in the liquid channels. For example, a structure for a purging operation is used widely in the field of ink jet printers and includes a purge cap and a purge pump. The purge cap covers the ejection ports of the nozzles and is connected to the purge pump. In this structure, while the purge pump is pumping liquid out of the nozzles, air bubbles are discharged. Such a structure is shown in FIG. 2 of U.S. Patent Application Publication No. US 2004/125189 A1 corresponding to Japanese Patent Application Laid-open No. 2004-142450, for example. FIGS. 1 and 2C of PCT International Publication No. WO 01/08888A1 (corresponding to Published Japanese Translation of PCT International Publication for Patent Application No. 2003-505281 and U.S. Pat. No. 6,705,704 B2) show a liquid ejection apparatus for ejecting liquid from chambers (pressure chambers) through nozzles by varying the volume of the chambers while circulating liquid continuously between the chambers and a liquid supply source by aid of a pump or the like.
In many cases, such liquid channels are complex or complicated in shape, and thus the air bubbles mixed in them are liable to remain in the corners of the liquid channels. Therefore, when liquid is pumped from nozzles, as disclosed in U.S. Patent Application Publication No. US2004/125189 A1, it is necessary to perform a number of purging operations, with the result that liquid is wasted. The liquid ejection apparatus disclosed in WO 01/08888A1 circulates liquid continuously through the liquid channels. If the liquid is circulated under high pressure, the nozzle menisci may be broken, and thus liquid may leak from the nozzles the ejection through which is not programmed to do so. If the liquid is circulated under low pressure, it is not possible to reliably discharge the air bubbles in the liquid channels.